The present invention relates to a system for treating substrates by plasma and, more particularly, to a system for generating uniform plasma for processing a substrate having two major surfaces.
In the manufacture of many electronic components, such as integrated circuits, there is a need to deposit metallic films on substrates. Materials such as copper may be deposited on ceramic or glass substrates and then etched or otherwise fabricated into the electrical circuits and/or components.
In the field of plasma deposition, an atom may be displaced from the surface of a target connected to a cathode by a process called sputtering or sputter deposition. In this process, the target is most often constructed of electrically conductive material such as copper or the like. The cathode to which the target is attached is subjected to a relatively high voltage, either DC or radio frequency, in an inert atmosphere such as argon. The inert gas is ionized, forming an excited gaseous state (plasma) from which positive ions escape to bombard the exposed surface of the target. By momentum transfer, the atoms or clusters of atoms of the target material are thereby dislodged. It is this dislodging of the target atoms that is known as sputtering. By repeating this process, a number of these primarily neutral atoms move through the space in front of the target, in a relatively high vacuum. Eventually these atoms strike and condense on the surface of a receiver, known as a sample or substrate, which is generally in close proximity to the target. A coating of atomic or molecular layers of target material can thus be built on the substrate. The coating, which is generally less than 1 .mu.m, is called a thin film. It is generally sufficient for the metallization of integrated circuits.
Through holes or viaduct holes (commonly called vias) are paths for electrical interconnections between a first-level conductive pattern and a second or higher-level conductive pattern. In order to electrically connect circuits on different substrate levels to each other, precious metal (e.g., palladium) seeding and electroless metal deposition have been used to coat the walls of the vias, often followed by electroplating. Most recently, however, plasma technology has been applied to this problem.
In the field of plasma processing of substrates for use as printed circuit boards and cards, nonuniformity of a plasma field can result in nonuniform etching, nonuniform deposition and/or nonuniform cleaning of via holes and through holes, the latter being referred to as desmearing. In the etching process, for example, a more intense plasma field in the center of a board results in a higher etching rate for that portion of the board, whereas a relatively sparse plasma density at the edges of the board results in a proportionally and predictably low etch rate for those sections. It has been found that nonuniformity of an electric field in proximity to an electrically floating printed circuit board results in nonuniform plasma treatment thereof.
Normally, oppositely charged electrodes are provided in a vacuum chamber to initiate a plasma reaction. Such apparatus is shown, for example, in copending patent application, Ser. No. 587,098 filed Mar. 7, 1984 for "Shield for Improved Magnetron Sputter Deposition into Surface Recesses," and assigned to the present assignee.
For desmearing via holes and through holes, one technique is disclosed in U.S. Pat. No. 4,230,553, issued to Bartlett, et al. This technique uses plasma etching wherein the conductive surface layers of drilled boards are themselves the electrodes that help generate plasma. The plasma forms directly within the holes to remove the smear. A radio frequency (r.f.) generator is electrically connected to one surface of each of the boards being processed. The other surface of each of the boards is grounded. The plasma generated in this system is thus present only in the through holes. One r.f. source supplies power to the system.
It should be noted, however, that, while in a uniform electric field relatively low radio frequencies can drive ions through the substrate's through holes, this practice used in a nonuniform electric field has the opposite effect: nonuniformity of the etching process is exacerbated.
U.S. Pat. No. 4,285,800, issued to Welty, discloses a gas plasma reactor for treatment of printed circuit boards. Included in the reactor is a rack assembly having a plurality of spaced apart bars for holding the substrates. A pair of electrodes is positioned outside the rack assembly. The rack assembly is maintained at ground potential and the electrodes are energized with r.f. energy to form plasma between the electrodes and the rack bars.
The aforementioned references are not appropriate for etching large substrates on both major surfaces or for generating uniform plasma fields over a large area.
It would be advantageous to provide a plasma reactor system for generating uniform plasma fields.
It would further be advantageous to provide a plasma system for uniformly processing the major surfaces as well as the via holes and through holes of large substrates.
Moreover, it would be advantageous to provide a plasma reactor in which the substrate is maintained at a fixed voltage potential relative to r.f. power supplies.
It would also be advantageous to maintain a substrate to be plasma processed at zero volts in a reactor system.
It would further be advantageous to provide a plasma reactor in which two r.f. power sources are out of phase with respect to one another to attain a more uniform plasma field.
It would further be advantageous for the plasma reactor r.f. power sources to be 180.degree. out of phase with one another.